Easy Joule Thief Circuit

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Introduction: Easy Joule Thief Circuit

About: Creating DIY projects

A joule thief takes a low DC voltage and boosts it up to a higher voltage. You can even take a battery that other devices will consider to be drained and get a bit more of the energy out of it. You won't get much current from it, but you can get enough to power a 3 volt LED with a battery that's drained to less than a volt. That's why it's referred to as a joule thief. It's almost like stealing energy from a dead battery. By the way, some people call it a zombie circuit for this same reason.

If you're new to building electronics, this is a good circuit for practice. The one I made for this Instructable looks ugly, but it still works. The schematic that I'm following is a basic one that I got from Wikipedia. (https://en.wikipedia.org/wiki/Joule_thief) It uses 4 components plus a battery. You can see some schematic reference pictures in step 10 of this Instructable.

If you would like to see a video version of this instructable, you can check that out here: https://youtu.be/nBn4JEvv93c

Supplies

Step 1: Build a Toroid Inductor

I start by making a toroid inductor. Using 2 wires, I wrap them around the inductor core until the core is filled. The wires I used are 22 gauge, but they can be even thinner. Wrap them both the same number of times. I used 2 different colors of wires to help me keep track of which is which. (In these pictures I used red and black, but I switched to red and green.)

After wrapping them, I mix up some epoxy and coat the toroid with it. This isn't absolutely necessary, but it helps to hold the wires in place on the toroid.

Step 2: Mount the Toroid Inductor - First Component

When the epoxy sets, I stripped back the wires and soldered the toroid inductor to a perfboard. Keep track of which wires are which. I have the 4 connections labeled in the picture.

Step 3: 1K Ohm Resistor - Second Component

The second part that I'm adding is a 1k resistor. Connect G1 from the toroid inductor to one side of the resistor, and it does not matter which side of the resistor we connect it to. The other side of the resistor needs to connect to R2 of the toroid inductor.

Add a (preferably red) wire for the battery positive. It also connects to the resistor at the same point as R2.

Step 4: 2N2222 Transistor - Third Component

The 3rd part is a transistor. I used a 2N2222 transistor, but you could instead use a 2N3904, BC547B, 2SC2500, BC337, 2N2222, 2N4401 or other NPN transistor, if that's what you have. Place it on the perfboard with the flat side facing away from the toroid. For now I'm just solder one of the leads to hold it to the board.

Step 5: Transistor Collector and Base

Now it's time to solder R1 of the toroid inductor to it's destination. Route it around to meet up with the collector of the transistor.

And G2, route that to connect with the base of the transistor. It's the middle pin.

Step 6: LED - Forth Component

The 4th part is the LED. The LED that I used is one that I salvaged from another board, so the leads are already cut short. The emitter of the transistor will connect to the negative pin of the LED. Since the LED leads are short, I created a path with solder.

I also connected a black wire to this same point for the battery negative.

Step 7: LED + Connection

The LED positive lead needs to connect to the same wire as R1 from the toroid inductor and the collector of the transistor. It also needs to pass over the wire for G2 without connecting to it. I used a jumper wire on the other side of the perfboard to do this.

Step 8: (Optional) Semi-Universal Battery Connector

This next step isn't necessary for the circuit to work, but it will give you a convenient way to connect to a variety of batteries. After sliding a disk magnet onto each of the power wires, solder a thumbtack onto each of those wire.

Step 9: Demo

Now it's complete so let's do a couple demonstrations with AA batteries. First I'm using a battery that's not new, but also not dead. You can see that it lights up the LED. Next I use a battery that is drained, and most devices consider it dead. And again, the LED lights up. With this circuit the toroid inductor may buzz a bit, so don't worry if it makes a high pitched sound. Thank you for checking out this Instructable!

Step 10: Schematic Reference Pictures

1 Person Made This Project!

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18 Comments

0
jimluschen
jimluschen

3 months ago

It's more properly called a toroidal transformer.

0
How Do You - DIY
How Do You - DIY

Reply 3 months ago

I think joule thief is kind of like a slang name for it.

0
jimluschen
jimluschen

Reply 3 months ago

Yes, it's a play on the term "jewel thief"

0
jrobinette
jrobinette

Reply 3 months ago

If it's a transformer, how does it boost voltage since the windings are the same? (I'm pretty sure I displaying my ignorance here).

1
jimluschen
jimluschen

Reply 3 months ago

First, an inductor is a two-terminal electrical component. Since this thing has four terminals, it can't be an inductor. Also, the dots and dashed lines indicate "transformer". That is what is drawn in the schematic.

Of course, a transformer is just two inductors with linked magnetic flux. So while your component isn't "an inductor", it is "two inductors". Or really "two coupled inductors".

Confusion arises because common transformers step voltages up (or down). But a transformer can have an equal number of windings in the primary and secondary, in which case it does not increase or decrease voltage. This type of transformer is often called an isolation transformer.

So you are right, this transformer would not boost voltage. But it would couple the two sides of the circuit, while providing a phase shift between them.

In this circuit I suspect that phase shift is the most important feature of the transformer, allowing the collector and base to see the same voltage vs. time signal, but with a phase shift between them. That phase shift is what causes the circuit to oscillate.

You might also think of the transformer as the feedback path that provides the necessary phase-shifted feedback from the collector (output) back to the base (input).

Oscillators are tricky circuits, and in my opinion their designers are magicians (especially those YIG oscillator guys). But in any case, LTSpice says your circuit works. I am attaching a screenshot of the analysis I ran of it. The inductances of the transformer is just a wild guess, so the oscialltion frequency is also.

Jim

2021-02-21_10-56-48.png
0
jrobinette
jrobinette

Reply 3 months ago

Many thanks for this. It certainly more items to learn about.

0
Techhlp
Techhlp

3 months ago

I can’t help but notice the perf board is from Radio Shack, I’m sure I have one of those very types in my box / drawers of hardware, also the types that replicate the format of solderless proto boards. The idea was, if anyone put the time in designing and tweaking a design, ‘solderless breadboard,’ until it’s just the way it ought to be, simply transfer the project component for component using the same layout. Then solder for permanence.
Keep up supporting the Radio Shack faith.

0
How Do You - DIY
How Do You - DIY

Reply 3 months ago

I used to wonder why they would make solder boards with the same layout as breadboards, but your explanation is perfect. It just makes sense. Thanks for sharing!

0
Pete Buxton
Pete Buxton

3 months ago

This is a clear and simple introduction to the magic of inductors.

0
charlessenf-gm
charlessenf-gm

Reply 3 months ago

Are there any practical applications for tis simple circuit? I mean other than getting an LED to light off a discarded battery that no longer has the energy to power the device it came out of?

0
How Do You - DIY
How Do You - DIY

Reply 3 months ago

That's a question. I think the main purpose for this specific circuit is educational. It's a good way to teach about and demonstrate inductors. It's also a good circuit for people to expand on as their education/experience grows. It's a basic DC voltage booster, so it can be used where a DC voltage needs to be boosted. But there are more complicated circuits that are usually better for that.

So I guess you could say this is more of a "build for fun/practice" type of circuit.

0
How Do You - DIY
How Do You - DIY

Reply 3 months ago

Thanks! It's a good circuit for learning that.

0
JohnC430
JohnC430

3 months ago

cool and simple. nice.

0
juleshartslief
juleshartslief

Question 3 months ago

Schematic for simple flashing led panel light

0
badger4
badger4

3 months ago on Step 8

This is a really clever way to make the magnetic battery connector.

0
How Do You - DIY
How Do You - DIY

Reply 3 months ago

Thanks! I figured out this trick for a different project, decided to use it here too.

1
CurtR
CurtR

3 months ago

Have you considered revising the circuit to include a light controlled circuit to turn it off during the day? That would be a handy feature.

0
How Do You - DIY
How Do You - DIY

Reply 3 months ago

I haven't, but that's a really good idea! It should be easy to do. Thanks for sharing your idea!